Gulpfile.js with browserify and external vendor file

Gulpfile.js

var gulp = require('gulp');
var sourcemaps = require('gulp-sourcemaps');
var source = require('vinyl-source-stream');
var buffer = require('vinyl-buffer');
var browserify = require('browserify');
var watchify = require('watchify');
var babelify = require('babelify');
var assign = require('lodash.assign');
var gutil = require('gulp-util');
var chalk = require('chalk');
var size = require('gulp-size');
var through = require('through');

const VENDORS = ['angular', 'jquery', 'bootstrap'];

function compile(watch) {
    var browserifyOptions = {
        entries: ['./build/src/index.js'],
        debug: true,
        external: VENDORS,
        bundleExternal: false
    };
    
    var options = assign({}, watchify.args, browserifyOptions);
    
    var bundler = browserify(options)
        .transform(babelify, {presets: ["es2015"]})
        .transform({global: true}, 'browserify-shim')
    
    function rebundle() {
        bundler.bundle()
            .on('error', function (error) {
                console.error(error.message);
                this.emit('end');
            })
            .pipe(source('app.js'))
            .pipe(buffer())
            .pipe(sourcemaps.init({ loadMaps: true }))
            .pipe(sourcemaps.write('./maps'))
            .pipe(size({showFiles: true}))
            .pipe(gulp.dest('./dist/src'));
    }
    
    if (watch) {
        bundler = watchify(bundler);
        bundler.on('update', rebundle);
    }
    
    bundler.on('log', function (log) {
        gutil.log(chalk.green('Browserify') + ': ' + log);
    });
    
    return rebundle();
}

gulp.task('build:app', function () {
    return compile();
});

gulp.task('watch', function () {
    return compile(true);
});

gulp.task('less', function () {
    return gulp.src('./build/less/main.less')
});

gulp.task('build:vendor', function () {
    var bundle = browserify({
        debug: true
    });
    
    bundle.transform(function(file) {
        if (file.indexOf('bootstrap') === -1) {
            return through();
        }

        var data = '';
        return through(write, end);

        function write (buf) { data += buf }
        function end () {
            data = "var jQuery = require('jquery');\n\n" + data;
            this.queue(data);
            this.queue(null);
        }
    }, {global: true});
    
    VENDORS.forEach(function (lib) {
        bundle.require(lib);
    });
    
    return bundle.bundle()
        .pipe(source('vendor.js'))
        .pipe(buffer())
        .pipe(sourcemaps.init({ loadMaps: true }))
        .pipe(sourcemaps.write('./maps'))
        .pipe(size({showFiles: true}))
        .pipe(gulp.dest('./dist/src/'));
});